Human Microbiome May Be Seeded Before BirthWe are each home to about 100 trillion bacteria, which we carry with us from birth till death. But whenJuliette C. Madan was trained as a neonatologist in the mid-2000s, her teachers told her in nouncertain terms that we only acquire those bacteria after we are born. “It was clear as day, we weretold, that fetuses were sterile,” she said.

Dr. Madan is now an assistant professor of pediatrics at the Geisel School of Medicine at Dartmouth,and she’s come to a decidedly different view on the matter. “I think that the tenet that healthy fetusesare sterile is insane,” she said.

Dr. Madan and a number of other researchers are now convinced mothers seed their fetuses withmicrobes during pregnancy. They argue that this early inoculation may be important to the long-termhealth of babies. And manipulating these fetal microbes could open up new ways to treat medicalconditions ranging from pre-term labor to allergies.

In 1900, the French pediatrician Henry Tissier declared unborn babies bacteria-free. Only when theystarted their journey down through the birth canal did they begin to get covered with microbes. Thenewborns then acquired more as they were handled and nursed.

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Credit David Fairfield/Getty Images

“This was considered a kind of scientific dogma,” said Esther Jiménez Quintana of ComplutenseUniversity of Madrid.

This dogma gained strength from studies on babies born prematurely. Infections are a major riskfactor in early labor. Many researchers saw this as evidence that the only bacteria in the uterus weredangerous ones.

But scientists came to this conclusion without finding out whether healthy fetuses had bacteria, too. “Itbecame a self-fulfilling prophecy,” said Dr. Madan.

That has started to change in the past few years. In 2010, Josef Neu, a University of Floridapediatrician, examined the first stool from newborn babies, before they had their first meal. He found adiversity of bacteria in the stool, whether the babies were born on time or born prematurely.

“When we first saw this, we though it was an artifact,” said Dr. Neu. If the fetuses were indeed sterile,their stool should have been germ-free. But in follow-up studies, he has gotten the same results.

Other scientists have also found evidence indicating that healthy fetuses pick up bacteria in the womb.Dr. Quintana and her colleagues have found bacteria in the amniotic fluid of healthy babies, as well asin umbilical cord blood and placentas.

If other animals are any guide, we shouldn’t be surprised if human fetuses are laced with bacteria. Inan essay published last week in the journal PLOS Biology, Seth R. Bordenstein and Lisa J. Funkhouserof Vanderbilt University observed that mothers transmitting bacteria to their offspring is the rulerather than the exception in the animal kingdom. Studying other species may give scientists cluesabout how human mothers inoculate their unborn children.

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Carl Zimmer Credit Earl Wilson/The New York Times

One open question is the route that bacteria take from mothers to their fetuses. A number ofresearchers suspect that immune cells in the mother’s intestines swallow up bacteria there and ferrythem into the bloodstream, where they eventually wind up in the uterus.

It’s also not clear whether mothers deliver a random collection of species or a special set that are

MatterMatter. It’s the stuff ofeverything — large andsmall.

beneficial to them. Studies on children and adults have shown that our resident bacteria — collectivelyknown as the microbiome — help us in many ways. They digest compounds in our food that wouldotherwise be indigestible.

Beneficial bacteria also help tutor the immune system, so that it attacks pathogens withoutoverreacting and damaging the body itself. The microbiome can even fend off disease-causing bacteria.

Dr. Neu and other pediatricians are now investigating whether the microbiome helps fetuses beforebirth. He speculates that a healthy supply of bacteria in a fetus can reduce the chances of prematurebirth. If harmful bacteria manage to slip past those defenses, they may trigger an immune reaction thatis sensed by the mother, prompting her to go into labor.

As scientists investigate the microbiome, they are also exploring ways of manipulating it to treatdisorders ranging from gut infections to autoimmune disorders. Dr. Neu hopes it may be possiblesomeday to bring the same medical help to fetuses.

“We might provide mothers with a microbial cocktail,” he said. The bacteria would pass from a motherto her fetus. Doctors might prescribe certain species to protect the fetus from infections, warding offearly labor. Nurturing the fetal microbiome could help babies in other ways, like boosting theirimmune system.

Some scientists don’t think the evidence supports these ideas, though. Bacteria in fetuses may not haveany special role to play in their health. “It could just be part of the vulnerabilities that pregnancy poseson the maternal body,” said Maria Dominguez-Bello, an associate professor at N.Y.U. Langone MedicalCenter.

But figuring out which explanation is right will demand the careful study of healthy fetuses —something that has only barely begun.

“The frontier is ahead of us,” said Dr. Bordenstein.

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